The present invention relates to volume change monitors for use in detecting changes of a volume enclosed thereby which would typically be that of a section of a human torso and, more particularly, to determine volume changes during respiration as an indicator of the occurrence of apneas in the human being monitored.
As is becoming increasingly recognized, interference with sleep in humans due to irregularities in bodily systems is emerging as a causative factor in an increasing number of human maladies. A corollary of this recognition is the further recognition of the increasing importance of analyses of such irregularities.
A known important irregularity is the temporary cessation of breathing during sleep, i.e. a sleep apnea. Three kinds of such sleep apneas are generally recognized in humans, these being central apneas due to problems occurring in the central nervous system, obstructive apneas due to blockages of the air passageways involved in respiration, and mixed apneas involving both of these previous kinds of apneas.
A desirable method of determining the occurrence of obstructive apnea is based on the measurement of volume changes in the human torso due to respiration as an analog of breathing effort changes. This method avoids the need to place something directly into the subject's esophagus to directly measure structural pressure changes, which such a subject will find to be quite uncomfortable during installation and may not tolerate it at all.
There are known methods for monitoring such volume changes along the torso of a subject, including impedance pneumography and inductive plethysmography which are expensive methods and which to some extent pick up unwanted signals such as those accompanying cardiac events. Another known method is the use of a tube about the torso at a selected location having air in the interior space thereof, and a pressure monitoring apparatus connected to that space. The volume changes of the torso at the location of the tube lead to air pressure changes in its interior space which are monitored by the pressure monitoring apparatus.
The use of a tube containing air in its interior space and a pressure monitor provides a cheaper means of monitoring volume changes in the subject's torso than does the equipment used in the first two methods mentioned above. However, there are also difficulties with the use of such a tube-based system which must be overcome. The tube used must not be pinched closed by the weight of the torso of the subject against the tube and the support for the subject and the tube to assure that pressure changes in the tube interior, corresponding to volume changes in the subject's torso due to respiration, are communicated to the pressure gauge. One means of providing that assurance is to inflate the tube with sufficient pressure in the interior thereof so that it cannot be pinched off by the weight of the subject. However, this pressure makes the tube uncomfortable for the subject to wear.
Another means is to have a pair of resilient material strips, or tapes, provided within the tube with a space therebetween so that a passageway is maintained by the material even though compressed by the weight of the subject's torso. However, a satisfactory tube must have walls that are rather thin so that the tube remains sensitive to even small volume changes of the torso of the subject for reasons to be described below. As a result, the tube wall material must be subject to being rather easily deformed. In these circumstances, the tossing and turning of a subject during sleep with such a tube thereabout raises a concern that the two strip portions, joined by portions of the thin wall of the tube, may be repeatedly rolled over one another into a fairly tight twist.
As the result of such twisting, respiration volume changes, reflected by corresponding pressure changes in portions of the tube on one side of such a twist, may no longer be communicated to the other side of the twist which may be the side connected to the pressure monitoring apparatus. Thus, there is a desire to have a volume change monitoring apparatus based on varying air pressure occurring inside a tube placed about the torso of a subject in response to volume changes in that torso at that location configured to assure that pressure changes occurring in any one part of the tube will be communicated throughout and so to a pressure gauge connected thereto.